你文档写的runloop 好像是在主线程执行的runloop

NSMutableArray *mutableIndexPathsToBePrecached = self.fd_allIndexPathsToBePrecached.mutableCopy;
CFRunLoopObserverRef observer = CFRunLoopObserverCreateWithHandler(kCFAllocatorDefault, kCFRunLoopBeforeWaiting, true, 0, ^(CFRunLoopObserverRef observer, CFRunLoopActivity _) {
if (mutableIndexPathsToBePrecached.count == 0) {
CFRunLoopRemoveObserver(runLoop, observer, runLoopMode);
CFRelease(observer); // 注意释放，否则会造成内存泄露
return;
}
NSIndexPath *indexPath = mutableIndexPathsToBePrecached.firstObject;
[mutableIndexPathsToBePrecached removeObject:indexPath];
[self performSelector:@selector(fd_precacheIndexPathIfNeeded:)
onThread:[NSThread mainThread]
withObject:indexPath
waitUntilDone:NO
modes:@[NSDefaultRunLoopMode]];
});

:[NSThread mainThread] 这不是主线程执行的runloop吗